The present invention relates to a method for manufacturing optical cables, particularly for manufacturing optical cables containing optical fibres loosely placed and a cable so manufactured.
More specifically, one aspect of this invention concerns a method for controlling the amount of optical fibre in an elongated jacket suitable for containing the optical fibre, specifically a tubular element, typically made of plastic.
Additional aspects of this invention concern an optical element, consisting of a tubular element containing one or more optical fibres of controlled length, a method for manufacturing this optical element and a cable comprising this optical element.
Currently, the optical fibre manufacturing method consists in loosely inserting one or more optical fibres inside a plastic tube to form the so-called xe2x80x9coptical corexe2x80x9d of the cable. This element, also known as xe2x80x9cloose tubexe2x80x9d or xe2x80x9cbuffer tubexe2x80x9d, can then be used, in different configurations, to manufacture optical cables, singly or in groups of several tubes. These tubes can contain either single optical fibres, or groups of optical fibres grouped in one or more bundles, or one or more ribbons. Typically, the tubes also contain a filler, e.g. grease, to prevent water from accidentally seeping into the tube and propagating longitudinally inside.
The length of the fibres in the tubes (single, bundles or ribbons) can be equal to, longer or shorts than the (axial) length of the tube. For the purpose of this description, the difference in length between fibre and tube will conventionally be called xe2x80x9cexcess fibrexe2x80x9d. In particular, when the fibre is longer than the tube containing it, the term xe2x80x9cpositive excess fibrexe2x80x9d will be used. On the contrary, when the fibre is shorter than the tube containing it, the term xe2x80x9cnegative excess fibrexe2x80x9d will be used. Finally, the term zero excess fibre will be used to indicate that the length of the fibre is substantially the same as that of the tube containing it.
Typically, the difference in length of the fibre in the tube allows cable structure stretching and shrinking caused by, for example, thermal variations or mechanical handling, to avoid cable length variations from affecting the fibre. In fact, unlike polymers, the vitreous material forming the optical fibre is not very sensitive to the temperature variations that the cable is subjected to during use, but it can present problems if mechanically stretched. Consequently, the length of the fibre in the tube should generally allow the tube to follow the length variations associated with the stresses (mechanical and thermal) it is subjected to, without imposing undesired mechanical traction or other attenuation-causing phenomena on the fibre. For example, positive excess fibre is suitable for high temperature environment or overhead cable optical fibre applications (subject to stretching due to own weight) to compensate for the structural stretching of the cable in order to allow the fibre to follow such stretches without suffering undesired stretches. This ensures that the fibre can follow the stretching without being undesirably stretched itself. On the other hand, for low temperature environment applications of an optical cable, the structural contraction of such cable tends to increase the excess fibre value. In this case, if a positive excess fibre were used, the additional increase of the value could cause excessive fibre bending in the tube, with the risk of inducing signal attenuation. In these cases, the use of negative excess fibre may be suitable.
Typically in the production of loose optical cores, the plastic material is extruded at high temperature around the fibres to form a tube which, once cooled, is wound on special reels.
One method for making loose cables and controlling excess fibre is described in U.S. Pat. No. 4,414,165 by Oestreich et al. This patent describes a method and equipment for forming an optical transmission element with loose optical fibres in a tubular jacket containing filling material.
Another method for producing loose cables and controlling fibre length, with respect to the length of the tube containing the fibre, is described in U.S. Pat. No. 5,372,757 by Schneider et al. In particular, as described in this patent, a traction force at high temperature is applied to the plastic tube and to the optical fibres. The tube is then cooled, maintaining the traction force. The applicant, however, has observed that in the lapse of time between tube production and subsequent application, e.g. to make an optical cable employing this tube, undesired and unforeseeable longitudinal shrinking can occur, with consequent uncontrollable variations of the ratio between tube length and fibre length.
Consequently, as observed by the applicant, excess fibre variations must be controlled both during the excess fibre controlling stage on the extrusion line and during the period from production of the tube, which is typically wound on a reel at the end of the production process, to its subsequent employment for making the cable. Typically, storage times (i.e. the time in which the tubes are wound on the reel before being used to make the cable) vary from several hours to approximately one week.
In particular, the applicant has observed that once the optical cores, made according to known techniques, are collected on- a reel, the plastic material forming the tube tends to additionally settle and, in particular, shrink. This settling generally cannot be foreseen; however, it usually causes additional tube shrinking leading to uncontrollable variationsxe2x80x94usually increasesxe2x80x94of the set excess fibre values.
The shrinking observed by the applicant in some cases results in sizes comparable to the excess fibre value set in production, with the result of substantially modifying the final excess fibre value and creating problems in the subsequent use of the tube in making the optical cables.
In particular, the applicant has observed that, at high production speeds, the tube is typically wound on the reel in random crossed turns. This unorderly tube winding generates gaps randomly distributed on the tube skein collected on the reel. The tube may detensionate more easily near these gaps and shrink, while detensioning may be obstructed in other areas. This causes different, uncontrolled shrinking of the tubes wound on different reels and also along different lengths of the same tube wound on the same reel.
Having defined the problem, the applicant has found a solution to eliminate, or at least minimize, these length variations during the storage of plastic tubes containing optical fibres, by stretching the material forming the tube containing the optical fibres by a predefined amount.
One aspect of this invention, therefore, relates to a method for producing polymeric material tubes associated with one or more optical fibres comprising the following steps:
feeding at least one optical fibre along a path to an extruder;
extruding the polymeric material around said optical fibre to form the tube;
cooling the tube to a predefined final temperature; the following steps are performed during cooling:
applying a first traction force to the tube containing said optical fibre in a first section of said extrusion line;
applying a second traction force to said tube in a second section of said extrusion line, in substantial absence of congruence between said fibre and said tube since said second traction force is greater than said first traction force;
applying a third traction force to said tube in a third section of said extrusion line, said third traction force being less than said second traction force;
said second traction force will reduce tube longitudinal shrinking by at least 20% after a storage period of one week or longer immediately after extrusion, compared to a similar tube which is not stretched.
Preferably, such second traction force is applied at a tube temperature when the modulus of elasticity of the polymeric material is approximately 2000 Mpa, preferably between approximately 100 Mpa and approximately 2000 Mpa, or more preferably between approximately 300 Mpa and approximately 1500 Mpa.
Preferably, said final temperature is lower than approximately 40xc2x0 C., preferably approximately 20xc2x0 C.
The tube temperature variation during the application of the second traction force is limited.
Preferably, the temperature variation in the tube length subjected to the traction force is lower than approximately 10% of the total thermal gap subjected by the tube along the extrusion line. Preferably, the temperature variation in the tube length subjected to said second traction force is lower than approximately 20xc2x0 C. and more preferably lower than approximately 10xc2x0 C.
According to a preferred embodiment, said second traction force is predefined to induce a stretching of approximately 1% or more when the polimeric materials of the tube is polybutyleneterephthalate (PBT).
A second aspect of this invention relates to a polymer tube produced by extrusion comprising one or more optical fibres, allocated inside said tube, characterized by the fact that, during production, said tube is subjected to stretching so that its longitudinal shrinkage is 20% or more less than that of a tube which was not stretched, after a storage period of one week or longer immediately following extrusion.
Preferably, said tube should be made of polybutyleneterephthalate (PBT), polyethylene (PE) or polypropylene (PP) polymeric material.
Preferably, such stretching is approximately 1% or more for PBT polymeric material tubes.
In a further aspect, this invention relates to equipment for making a tube containing one or more optical fibres comprising:
an extruder suitable for producing a plastic material tube containing one or more optical fibres;
one or more cooling pools;
a stretching device suitable for applying traction to a length of said tube, with temperature variations in said tube length 10% lower than the total thermal head of the tube from extruder to ambient temperature.
In particular, this stretching device comprises a driving element and a braking element, located between the extruder and said driving element.
Said driving element can comprise a drive wheel or a pair of drive tracks. The braking element can comprise, in turn, a second drive wheel or pair of drive tracks where the tube is fed at a lower speed with respect to the speed of the tube at the driving element. Alternatively, such braking element can either be an idle wheel around which the tube is wound by one complete turn and to which a braking force is applied, or an inflatable sleeve with a substantially circular central opening in which the tube slides.
Preferably, said stretching device comprise a first drive wheel, set at a first revolution speed, and a second wheel, set at a slower revolution speed than the first.
Alternatively, said stretching device comprise:
a first drive device, suitable for stretching said tube at a first speed;
a second driving element, set at a speed substantially equal to the speed of said first drive device;
a third element, located between said two driving elements, suitable for applying a force directly perpendicular to said tube feeding direction on the length of the tube between the two driving elements.
Alternatively, the stretching device comprise:
a driving element;
a braking element, comprising two set of rollers between which the tube is fed; being such two set of rollers arranged alternatively at opposite ends with respect to the central tube axis so that the distance between the lower surface tangent of the upper set and the upper surface tangent of the lower set is smaller than the diameter of the tube by a certain value to cause predefined stretching of the tube.